Method of forming a transparent and gas-permeability decreasing coating to a paper or board web and a coating formulation for the method
The present invention relates to a method for forming a coating to a paper or board web. It is an object of the coating method according to the invention to provide paper or board with a barrier resisting the transmission of gases and vapours such as oxygen and water vapour as is required in the art of packaging, for instance.
It is a further object of the invention to provide a coating with the above-mentioned qualities that is also transparent.
It is still another object of the invention to provide a coating with the above-mentioned qualities that also can reduce the adherence risk of the web surfaces in a winded paper of board roll.
The invention also concerns a coating formulation capable of providing the above-described qualities when applied to a paper web.
As known, the gas barrier property required in the packaging industry is conventionally achieved by means of coating the paper or board web with a polymer film. Desirably, the thin-film coatings used herein are capable of preventing penetration of liquids, such as water, and oxygen, water vapour and aroma components as well as oils and greases through them.
Handling of scrapped packaging materials coated with a functional polymer film often involves a high cost due to the poor degrading and recycling properties of these materials.
On the other hand, it is known in the art that a moisture barrier property can be given to a paper web by means of coating the web with a polymer latex having a wax emulsion added thereto. It is also known that these coating components are selected from the group of conventional synthetic polymer lateses such as styrene butadiene, acrylate, styrene acrylate and polyvinyl acetate lateses. Generally, the fraction of polymer lateses in the solids of the coating mixture is very high. Examples of the above-described technology can be found in patent publications. For instance, GB Pat. No. 1,593,331 (filed by J. Vase, Kemi Oy) and FI Pat. Appl. No. 901,928 (filed by Neusiedler A G) teach the methods of said technology.
According to another published reference, the amount of wax dispersion that may conventionally be added to a latex polymer is advantageously maximally about 10 wt-%, while also significantly higher amounts are possible. The waxes most commonly used include paraffin wax, microcrystalline wax, polyethylene wax and a mixture of one of these waxes with at least one other type of wax. A coating formulation thus prepared gives an extremely hydrophobic coating. Of publication closely related to the art, reference can be made to the patent publication WO-A1-9605054 (International Paper Company, USA), in which the above techniques are discussed.
The techniques taught in cited publications are capable of rendering desired barrier properties to the web. However, as the drying of the web leaving the coating section of the paper machine elevates the temperature of the coating applied to the paper web, the adherence risk of the coating in the winding of the paper web becomes so high as to exclude the use of above-mentioned conventional coating formulations when the goal is to achieve a uniform and defect-free coating. A further disadvangae is, that the high fraction of polymer latex in the coating formulation complicates the reuse of the coated paper in pulping, because the amount of anionic contaminants rises excessively high.
It must also be noted that the use of wax emulsions in applications related to the invention lowers the surface energy level of the coating applied to the paper web. The reduced level of surface energy in the coating complicates and, in many cases, even makes it impossible to perform surface sizing with, e.g., starches. The use of waxes also deteriorates the printability qualities of the coating, particularly for water-based inks. The use of wax coatings offers the benefit that it lowers the adherence risk of the coating, the improvement obtained herein is frequently, however, insufficient unless some mineral filler or pigment additive is used.
From the prior art is further known that polymer chains, particularly starch, of natural origin may be added to the latex polymer during its preparation. Paper products which are coated with a polymer latex thus prepared are easier to repulp than paper grades coated with pure polymer latexes. An example of this technique can be found in the patent literature. The technique is taught in publication WO-A-93/11300 (Raision Tehtaat Oy Ab). While the technique can aid the pulping of the recycled paper product, it simultaneously compromises heavily the water-vapour barrier property of the product due to the hydrophilic character of the added starch.
On the other hand the prior art teaches that talc particles can be used in a coating formulations intended for coating a paper or board web. Paper grades parti-cularly suited for gravure printing are coated using a coating formulation containing a significant fraction of talc particles.
Talc particles are also used as a constituent of coating mixtures developed for the coating of release paper grades and of coating mixtures applied to corrugated board grades suitable for flexo printing. Examples of this latter type of technique can be found in the patent literature. For instance, patent publications JP 62-64038 (filed by Oji Paper Co.) and JP 52-118016 (filed by Toyo Ink Mfg. KK) teach the above-described type of technique.
The technology according to these prior-art methods results in good release properties and improved resistance against water vapour transmission. However, due to the high proportion of talc particles with respect to the proportion of polymer latex in the coating mixture, the water vapour barrier property required in foodstuff packages, for instance, cannot be attained.
It is an object of the present invention to overcome the above-described disadvantages and to provide a paper product with sufficient barrier properties, yet suitable for reuse in papermaking and, moreover, permitting dumping by composting.
The goal of the invention is achieved by virtue of applying a coating containing at least one polymer dispersion on a sheet of paper, board or similar cellulosic fiber based web in at least one coating step, wherein the polymer dispersion used contains extremely pure talc particles.
In the context of the present invention, the term extremely pure talc particles is used when reference is made to particles having a degree of purity of about 90-100%. Advantageously, about 90% of the particles are smaller than 40 xcexcm. Talc having said properties minimizes the need of dispersants in dispersing the talc for the coating paste. High dispersant levels in coating formulations intended to have barrier properties are not favoured.
Herein, the term polymer latex refers to a polymer dispersion having advantageously a 30-60% solids content and advantageously the glass transition point in the range of xe2x88x9220-+70xc2x0 C. for use in applications according to the invention. For use according to the invention, suitable polymer lateses are selected from the group of synthetic polymer lateses including styrene butadiene, acrylate, styrene acrylate and polyvinyl acetate lateses and polymer dispersions made from a biologically degrading polymer, as well as mixtures of said polymer dispersions.
Accordingly, the polymer dispersion can be a product made starting from a mixture of monomers containing vinyl acetate and at least one ester formed from acryl acid and/or methacryl acid with a lower alcohol (that is, methyl, ethyl, propyl or butyl alcohol) as the main components.
Alternatively, the method can be implemented using a product made starting from a mixture of monomers containing styrene and at least one ester formed from acryl acid and/or methacryl acid with a lower alcohol (that is, a methyl, ethyl, propyl or butyl alcohol) as the main components.
Alternatively, the polymer dispersion can be a product starting from a mixture containing at least one ester formed from acryl acid and/or methacryl acid with a lower alcohol (that is, a methyl, ethyl, propyl or butyl alcohol), and/or copolymers of said compounds, as the main components of the mixture.
In a preferred embodiment, the main component of the polymer dispersion used according to the invention is a biologically degrading polymer such as one based on starch.
The coating formulation according to the invention improves the protection of the paper web against gas and water vapour permeability; simultaneously the coating smooths out any possible roughness of the paper web, thus reducing the consumption of another kind of coating to be subsequently applied to the pre coated web.
The coating mixture prepared according to the invention can be used in coated paper grades principally including different types of packages and wrappers which have to exhibit certain barrier properties against moisture and water vapour penetration and often also protection against oil and grease penetration as well as transmission of gases such as oxygen, for instance.
According to another aspect of the invention, the coating mixture can also be applied on a polymer film known to cause sticking of adjacent web surfaces, whereby the winding of the coated paper without the risk of web adherence to the adjacent surface becomes possible.
The novel coating formulation may also be used for pre treating a paper web which subsequently is coated by an other type of coating intended to render other kinds of properties to the web such as the suitability to be heat sealed.
According to a preferred embodiment of the invention, the proportion of talc particles in the coating mixture is 30-80% of solids in the dry coating mixture. It was noticed that by using talk contents 80% or lower in the coating composition, the composition forms a totally transparent film. This is due to the total adhesion of the polymer and the pigment, whereby no light scattering voids and irregularities are remaining in the coating layer.
The coating mixture formulation used in the invention, in which the above-described polymer dispersion and said kind of extremely pure talc particles are the main components, may be additionally complemented with other pigment or mineral particles, for instance for increasing the opacity of the coating mixture. The addition of waxes and colours is also another possibility to adjust the properties of the coating composition of the invention. Among colours the water-based colours are one preferable source in colouring the composition.
The amount of other pigment or mineral components can be increased up to 30% of the coating mixture solids. Clay, calcium carbonate, titanium dioxide, gypsum and organic pigments can be used as such additional components. The amount of wax may be up to 20% of the coating mixture solids. The amount of colours can vary from 0 to 5% of the overall coating mixture solids.
The surface energy of the polymer film achieved using the coating composition according to the invention can be further regulated by a reaction with siloxanes and poly siloxanes. This can be used for regulating for instance the printability of the film. Also adhesion properties towards different surfaces can be effected by using said further components in the coating compositions.
The invention can be implemented in two alternative ways, the first technique (A) comprising dispersing the talc particles in the aqueous phase using only an anti foaming agent and sodium hydroxide as the additives of the dispersing process, followed by mixing the slurried talc dispersion with a polymer latex. The second technique (B) comprises dispersing the talc particles in a polymer latex using similar additives as mentioned above in very small quantities, complemented with a dispersant and, if required, a wetting agent.
The term coating is in the present context used when reference is made to a formulation suitable for application to a paper or board web so as to act on a paper product as a coating with barrier properties against the transmission of water, water vapour and oxygen, among others.
The coating mixture according to the invention can be applied to the web using conventional coating apparatuses developed for coating a paper or board web. Advantageously, the applied coat weight is 3-30 g/m2 as the solids of the coating mixture.